1. Field of the Invention
The present invention relates to an automatic transmission power control circuit provided with a function to detect an abnormality of a transmission power and, more particularly, to an automatic transmission power control circuit for a transmitter which requires a monitor of the transmission power in a base station of a mobile radio communication system.
2. Description of the Related Art
An allowable fluctuation of an output power of a base station in a mobile communication system is specified, for example, as a range from +20 to -50% with respect to a reference transmission power. In this case, an automatic transmission power control circuit is provided in the base station in order to absorb fluctuation in the transmission power due to allowable tolerances of circuitry and a change in circumstances. Additionally, an alarm signal may be generated when an abnormality occurs in a transmission power, that is, when the transmission power is out of an allowable range.
FIG. 1 is a circuit diagram of a conventional automatic transmission power control circuit. Hereinafter the automatic transmission power control circuit may be referred to as APC.
In the APC shown in FIG. 1, a modulated wave to be transmitted is transmitted from an antenna ANT via a variable gain amplifier 1, a power amplifier 2 and a directional coupler 3. A part of the transmission power is branched by the directional coupler 3, and is detected by a detector 4. An output of the detector 4 is compared with a reference voltage 6 by a comparator 5. When the output of the detector 4 is less than the reference voltage 6, a high-level signal "H" is output from the comparator 5. On the other hand, when the output of the detector 4 is greater than the reference voltage 6, a low-level signal "L" is output from the comparator 5. The output of the comparator 5 is input to an up-down counter 7. When the output of the comparator 5 is the high-level signal "H", the up-down counter 7 counts up a count value by the output of the comparator 5 when a transmission timing sampling clock T1 is input. When the output of the comparator 5 is the low-level signal "L", the up-down counter 7 counts down the count value by the output of the comparator 5 when the transmission timing sampling clock T1 is input. An output of the up-down counter 7 is converted into an analog signal by a D/A converter 8. The analog signal controls the variable gain amplifier 1. It should be noted that a loop formed by the variable gain amplifier 1, the power amplifier 2, the directional coupler 3, the detector 4, the comparator 5, the up-down counter 7 and the D/A converter 8 is referred to as an automatic transmission power control loop.
When the APC functions normally, a magnitude of amplification of the variable gain amplifier 1 is increased when a level of the voltage (hereinafter referred to as a detected voltage) output from the detector 4 is small. On the other hand, the magnitude of amplification of the variable gain amplifier 1 is decreased when the detected voltage is large. Accordingly, the transmission power of the antenna ANT can be set to a level specified by the reference voltage 6. In this case, a relationship between the reference voltage 6 (V.sub.REF) and the detected voltage is indicated in the left half of a graph of FIG. 2. That is, the voltage output from the detector 4 does not fluctuate too much, and is substantially equal to the reference voltage 6.
If the APC cannot control the transmission power while the transmission power is increasing, the detected voltage is continuously increased from the reference voltage 6 as indicated in the right half of the graph of FIG. 2. In such a case, the comparator 5 continuously outputs the low-level signal. On the other hand, if the APC cannot control the transmission power while the transmission power is decreasing, the detected voltage is continuously decreased from the reference voltage 6. In such a case, the comparator 5 continuously outputs the high-level signal.
A description will now be given of a generation of a transmission power abnormality alarm signal. A reference voltage 9 (V.sub.HALM) is provided for an upper limit of the transmission power, and a reference voltage 10 (V.sub.LALM) is provided for a lower limit of the transmission power. The reference voltage 9 is provided to a comparator 11, and the reference voltage 10 is provided to a comparator 12. The comparators 11 outputs a high-level signal "H" when the transmission power exceeds the upper limit, and the comparator 12 outputs a high-level signal "H" when the transmission power decreases below the lower limit. The outputs of the comparators 11 and 12 are sampled by respective sampling circuits 13 and 14 by the transmission timing sampling clock T1. The sampled outputs are supplied to an OR circuit 15. Accordingly, when an output of the OR circuit 15 is at the high level "H", this means that the transmission power exceeds the upper limit or decreases below the lower limit. This condition corresponds to an abnormal condition of the APC, and thus the output of the OR circuit is rendered to be a transmission power abnormality alarm signal. In the right half of the graph of FIG. 2, although the APC is not functioning, a circuit for detecting an abnormality of the transmission power is functioning. Thus, the output of the comparator 11 is the high-level signal "H" after a time t1 when the detected voltage exceeds the reference voltage 9 (V.sub.HALM). The output of the comparator 11 is sequentially output as the transmission power abnormality alarm signal via the sampling circuit 13 and the OR circuit 15.
When the transmission power is smaller than a desired power, the high-level signal "H" is output from the comparator 5 so as to increase the output of the up-down counter 7. Thereby, the output of the D/A converter 8 increases a magnitude of amplification of the variable gain amplifier 1 in response to the output level of the up-down counter 7. Under a normal condition, the transmission power from the antenna ANT can be controlled to a desired power level by increasing the magnitude of amplification of the variable gain amplifier 1. However, when a malfunction occurs in a transmission circuit of the power amplifier 2, the transmission power may not be increased to the desired power level even when the magnitude of amplification of the variable gain amplifier 1 is continuously increased by increasing the output of the up-down counter 7. In such a case, the comparator 5 continuously outputs the high-level signal "H". Thereby, the variable gain amplifier 1 is controlled in a range (saturated range) beyond an operatable range. As a result, a distortion may occur in the transmission wave output from the antenna ANT, an adjacent channel leakage power may be increased, or a modulation accuracy may be deteriorated. However, even in such an abnormal condition, the abnormality cannot be recognized since the detected voltage output from the detector 4 is between the reference voltage 9 (V.sub.HALM) and the reference voltage 10 (V.sub.LALM) as indicated by a dashed line 16 in FIG. 2.
As mentioned above, in the conventional technique, there is a problem in that an abnormality cannot be recognized even when the transmission power does not reach a desired value and is distorted due to a malfunction of a part of the transmission circuit since the abnormality is detected based on only a level of the transmission power. That is, the abnormality cannot be recognized as long as the voltage detected by the detector 4 is within the range between the upper limit and the lower limit.
Additionally, the conventional APC requires the comparing circuit to detect an abnormality in the transmission power. Particularly, when an attempt is taken to detect both an abnormal increase and an abnormal decrease in the transmission power, the two comparators 11 and 12 are required. Additionally, the reference voltages 9 and 10 must be adjusted to appropriate values.
Further, in the conventional technique, it takes a relatively long time until the transmission power is stabilized when a power is turned on. If the time is too long, it is possible that the transmission power abnormality detecting circuit is activated, resulting in generation of an alarm.